Utilizing procedures such as described in application Ser. No. 07/839,389, the environmental impact front the production of kraft pulp can be greatly minimized. Utilizing the techniques described in that application, and employing a few additional techniques according to the present invention, it is possible to "close" a bleaching system of a kraft pulping facility so that there is no or very small discharge of washing filtrates, or the like, into the environment; or it is possible to substantially close the bleaching system so that the only discharge from the washing/bleaching system is a small stream of wash filtrate used in smelt dissolution (or effectively treated so that there is essentially zero pollution associated with it when discharged).
Perhaps the most promising bleaching chemical for allowing chlorine to be eliminated while still achieving effective bleaching, is ozone. While ozone has been known as a bleaching agent for a long time, it has not been commercially used on an industrial scale in pulp production until very recently in view of recent rapid developments in machine and process technologies, because of long standing problems.
A major problem encountered in utilizing ozone is high ozone consumption. Ozone consumption may be unreasonable high if there are too many metallic ions, which react with the ozone, or if the ozone reacts with organic material in the liquid phase of the pulp rather than reacting with the pulp fibers. It is highly desirable to have the ozone react only with fibers so that the ozone consumption is minimized, making ozone bleaching practical as well as effective, and allowing "closing" of a bleach plant.
As disclosed in application Ser. No. 07/839,389, one way in which the volume of metallic ions can be decreased is by improving the washing after the acid stage, where the majority of the metallic ions are dissolved. Another way to reduce the load of metallic ions, according to parent application Ser. No. 07/861,387, is to utilize an electrical potential. The electrical potential can be applied over the washer, or the filtrate supplied to the washer, or even to the pulp prior to the ozone stage. A preferred way to apply the electric field is to have the filtrate being used as washwater to go through a chamber where there is a suitable electric potential. In this way the filtrate is cleaned of metallic ions before being used as wash water. The electrical potential can effect sufficient removal of ions so as to allow complete closing of the bleaching plant, or to provide that the amount of wash filtrate from the acid stage is so small that it is completely consumed in smelt dissolution, and cooking liquor preparation.
According to the present invention, the amount of organic material in the liquid phase is made non-attractive to the ozone by oxidizing the organic material. Since ozone is a strong oxidant, it is more likely to react with the fibers than with the already oxidized organic material in the liquid phase. It is perhaps not possible to oxidize all dissolved material in the liquid phase, but a considerable part can be oxidized with oxygen prior to applying the ozone. However there is always a significant amount of organic material in the wash filtrates used during washing of the pulp prior to the ozone stage so that if that organic material is oxidized, the ozone consumption in the following ozone bleaching stage can be significantly reduced.
According to one aspect of the present invention, a method of treating filtrates from bleach plants for bleaching cellulosic pulp, including utilizing at least one ozone, H.sub.2 O.sub.2, or oxygen bleaching stage, is provided. The method comprises the steps of: (a) Washing the pulp in a washer with a wash liquid prior to the ozone, H.sub.2 O.sub.2, or oxygen bleaching stage, the wash liquid including filtrate from another washing stage of a bleaching stage. (b) Oxidizing the organic material in the filtrate used as wash liquid in step (a) prior to its use in step (a) in order to make the organic material less attractive to bleaching chemical in the bleaching stage. And, (c) ozone, H.sub.2 O.sub.2, or oxygen bleaching the washed pulp in the at least one ozone, H.sub.2 O.sub.2, or oxygen bleaching stage. Step (b) may be practiced by indirectly heating the filtrate (as with already oxidized filtrate in the heat exchanger), and then reacting the heated filtrate with oxygen containing gas, such as oxygen carrier gas with residual ozone discharged from an ozone bleaching stage. Alkali can be added to the filtrate and oxygen during the reacting step if desired.
According to the invention, there is also the further step (d) of removing metallic ions from the filtrate so as to minimize the consumption of bleaching chemical in step (c), by passing the filtrate through a magnesium filter, such as a magnesium chip filter. Such a filter especially removes Mn, Fe, and Cu all of which consume bleaching chemicals. Mg, on the other hand, has a positive effect on bleaching.
All of the steps are preferably practiced with the pulp at a medium consistency, e.g. of about 5-18% (preferably about 8-15%).
Practicing the present invention, and the techniques from the parent application, a method of continuously kraft pulping and then bleaching comminuted cellulosic fibrous material may be provided which comprises the following steps: (a) Continuously digesting the material to produce pulp having a Kappa Number comparable to about 20 or below for softwood. (b) Subjecting the digested pulp to hot alkali extraction and/or oxygen bleaching so as to produce pulp having a Kappa Number comparable to about 10 or below for softwood. (c) Effecting bleaching of the pulp with non-chlorine containing bleaching chemicals to obtain a bleached pulp having properties comparable to CK pulp bleached with a DEDED sequence. (d) Effecting washing of the pulp with wash filtrates, and to produce wash filtrates, during the practice of step (c). And, (e) treating and recirculating the wash filtrates from step (d) so as to discharge no, or a small amount of, wash filtrates into the environment. Step (c) includes at least one ozone bleaching stage, and step (d) includes washing the pulp with wash filtrate just prior to the ozone bleaching stage; and comprising the further step (f) of oxidizing organic material in the wash filtrate prior to the washing of the pulp just prior to the ozone bleaching so as to make the organic material less attractive to ozone in the ozone bleaching stage. The bleaching sequences may be as described in the parent application, e.g. they may be selected from the group consisting essentially of OOAZEZPZ, OAZEZPZ, OOAZEZP, and OAZEZP, or AZEZPZ or AZEZP if oxygen bleaching is used to lower the Kappa Number to about 10.
Alternatively, step (e) may be the step of treating and recirculating all of the wash filtrates in step (d) so that the only wash filtrate discharged from the washing of step (d) is a small stream used for smelt dissolution (cooking liquor production). If all of the small stream cannot be taken to cooking liquor preparation (smelt dissolution and other associated processes), it is taken to black liquor evaporation. It can be taken to black liquor evaporation directly or by taking it to the brown stock washing plant.
It is the primary object of the present invention to minimize ozone consumption in the ozone bleaching of cellulosic pulp, and/or to produce a closed, or substantially closed, bleach plant in a pulp mill. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.